Digestion and Absorption

Alimentary Canal:

• The alimentary canal begins with an anterior opening – the mouth, and it opens out posteriorly through the anus.

  • The mouth leads to the ==buccal cavity== or oral cavity.
  • ==The oral cavity has a number of teeth and a muscular tongue.==
  • Each tooth is embedded in a ==socket of the jaw bone.==
  • ==This type of attachment is called thecodont.==
    • The majority of mammals including human beings form two sets of teeth during their life, ==a set of temporary milk or deciduous teeth replaced by a set of permanent or adult teeth.==
    • This type of dentition is called ==diphyodont.==
      • An adult human has ==32 permanent teeth== which are of four different types (Heterodont dentition), namely, incisors (I), canine (C), premolars (PM), and molars (M).
    • The arrangement of teeth in each half of the upper and lower jaw in the order I, C, PM, and M is represented by a dental formula which in humans is ==2123/2123.==
    • The hard chewing surface of the teeth, ==made up of enamel, helps in the mastication of food.==

  

• The tongue is a ==freely movable muscular organ attached to the floor of the oral cavity by the frenulum.==

  • The upper surface of the tongue has small projections called ==papillae, some of which bear taste buds.==

• The oral cavity leads into a ==short pharynx which serves as a common passage for food and air.==

  • The ==esophagus and the trachea (windpipe) open into the pharynx.==
  • A ==cartilaginous flap called epiglottis prevents the entry of food into the glottis== – opening of the windpipe – during swallowing.
  • The esophagus is a thin, long tube that extends posteriorly passing through the neck, thorax, and diaphragm, and leads to a ==‘J’ shaped bag-like structure called the stomach.==
  • A ==muscular sphincter (gastro-oesophageal) regulates the opening of the esophagus into the stomach.==

• The stomach, located in the upper left portion of the abdominal cavity, has four major parts – ==a cardiac portion into which the esophagus opens, a fundic region, a body (main central region), and a pyloric portion== which opens into the first part of the small intestine.

• The small intestine is distinguishable into three regions, a ‘==C’ shaped duodenum, a long coiled middle portion jejunum, and highly coiled ileum.==

  • The opening of the stomach into the duodenum is guarded by the ==pyloric sphincter.==
  • The ==ileum opens into the large intestine.==
  • It consists of the ==caecum, colon, and rectum.==
    • The caecum is a ==small blind sac== that hosts some symbiotic microorganisms.
    • A narrow finger-like ==tubular projection, the vermiform appendix== which is a ==vestigial organ==, arises from the caecum.
    • The caecum opens into the colon.
    • The colon is divided into four parts – ==an ascending, a transverse, descending part, and a sigmoid colon, the descending part== opens into the rectum which opens out through the anus.

  

• The wall of the alimentary canal from the esophagus to the rectum possesses four layers namely ==serosa, muscularis, submucosa, and mucosa.==

  • Serosa is the outermost layer and is made up of a ==thin mesothelium (epithelium of visceral organs)== with some connective tissues.
  • Muscularis is formed by smooth muscles usually arranged into an ==inner circular and an outer longitudinal layer.==
  • An oblique muscle layer may be present in some regions.
  • The ==submucosal layer is formed of loose connective tissues containing nerves, blood, and lymph vessels==.
  • In the duodenum, glands are also present in the sub-mucosa.
  • The innermost layer lining the ==lumen of the alimentary canal is the== ==mucosa.==
  • This layer forms irregular folds (rugae) in the stomach and ==small finger-like foldings called villi== in the small intestine.
  • The cells lining the villi produce numerous microscopic projections called ==microvilli giving a brush border appearance.==

  

• These modifications increase the surface area enormously.

  • Villi are supplied with a ==network of capillaries and a large lymph vessel called the lacteal.==
  • The mucosal epithelium has ==goblet cells that secrete mucus== that help in lubrication.
  • Mucosa also forms ==glands in the stomach (gastric glands) and crypts in between the bases of villi in the intestine (crypts of Lieberkuhn).==

• All four layers show modifications in different parts of the alimentary canal.

Digestive Glands:

• The digestive glands associated with the alimentary canal include the salivary glands, the liver, and the pancreas.

  • Saliva is mainly produced by three pairs of ==salivary glands, the parotids (cheek), the submaxillary/sub-mandibular (lower jaw), and the sub-linguals (below the tongue).==
  • These glands situated just outside the buccal cavity secrete salivary juice into the buccal cavity.
  • The ==liver is the largest gland of the body== weighing about 1.2 to 1.5 kg in an adult human.
    • It is situated in the abdominal cavity, just below the diaphragm, and has two lobes.
    • ==The hepatic lobules are the structural and functional units of the liver== containing hepatic cells arranged in the form of cords.
    • Each lobule is covered by a thin connective tissue sheath called ==the Glisson’s capsule.==
    • The bile secreted by the hepatic cells passes through the hepatic ducts and is stored and concentrated in a thin muscular sac called the ==gall bladder.==
  • The duct of the gall bladder ==(cystic duct)== along with the hepatic duct from the liver forms the common bile duct.
  • The ==bile duct and the pancreatic duct open together into the duodenum as the common hepato-pancreatic duct== which is guarded by a sphincter called the ==sphincter of Oddi.==
  • The pancreas is a compound (both exocrine and endocrine) elongated organ situated between ==the limbs of the ‘C’ shaped duodenum.==
  • The exocrine portion secretes an alkaline pancreatic juice containing enzymes and the endocrine portion secretes hormones, ==insulin, and glucagon.==

  

Digestion of Food:

• The process of digestion is accomplished by mechanical and chemical processes.

  • The buccal cavity performs two major functions, ==mastication of food and facilitation of swallowing.==
  • The teeth and the tongue with the help of saliva ==masticate and mix up the food== thoroughly.
  • Mucus in saliva helps in ==lubricating and adhering the masticated food particles into a bolus.==
    • The bolus is then conveyed into the pharynx and then into the esophagus by ==swallowing or deglutition.==
    • The bolus further passes down through the esophagus by successive ==waves of muscular contractions called peristalsis.==
    • The gastro-oesophageal sphincter controls the passage of food into the stomach.
    • The saliva secreted into the oral cavity contains electrolytes and enzymes, salivary amylase, and lysozyme.
    • The chemical process of digestion is initiated in the oral cavity by the hydrolytic action of the carbohydrate-splitting enzyme, salivary amylase.
    • About ==30 percent of starch is hydrolyzed here by this enzyme (optimum pH 6.8) into a disaccharide – maltose.==
    • ==Lysozyme present in saliva acts as an antibacterial agent that prevents infections.==

  

• The mucosa of the stomach has gastric glands.

• Gastric glands have three major types of cells namely -

  • Mucus neck cells that secrete mucus
  • ==Peptic or chief cells== ==which secrete the proenzyme pepsinogen==
  • ==Parietal or oxyntic cells== ==secrete HCl and intrinsic factors (factor essential for the absorption of== ==vitamin B12====).==

• The stomach stores the food for 4-5 hours.

  • The food mixes thoroughly with the acidic gastric juice of the stomach by the churning movements of its muscular wall and is called the ==chyme.==
  • The proenzyme pepsinogen, on exposure to hydrochloric acid, gets converted into the active enzyme pepsin, ==the proteolytic enzyme of the stomach.==
  • ==Pepsin converts proteins into proteoses and peptones (peptides).==
  • The mucus and bicarbonates present in gastric juice play an important role in ==lubrication and protection of the mucosal epithelium== from excoriation by highly concentrated hydrochloric acid.
  • HCl provides the acidic pH (pH 1.8) ==optimal for pepsins==.
  • ==Rennin== ==is a proteolytic enzyme found in the gastric juice of infants which helps in the digestion of milk proteins.==
  • Small amounts of lipases are also secreted by gastric glands.
  • Various types of movements are generated by the muscularis layer of the small intestine.
    • These movements help in a thorough mixing up of the food with various secretions in the intestine and thereby ==facilitate digestion.==
  • The ==bile, pancreatic juice, and intestinal juice== are the secretions released into the small intestine.
    • Pancreatic juice and bile are released through the hepato-pancreatic duct.
    • The pancreatic juice contains inactive enzymes – ==trypsinogen, chymotrypsinogen, procarboxypeptidases, amylases, lipases, and nucleases.==
    • ==Trypsinogen is activated by an enzyme, enterokinase==, secreted by the intestinal mucosa into active trypsin, which in turn activates the other enzymes in the pancreatic juice.
    • The bile released into the duodenum contains bile pigments (==bilirubin and biliverdin==), ==bile salts, cholesterol, and phospholipids but no enzymes.==
    • Bile helps ==in the emulsification of fats,== i.e., breaking down the fats into very small micelles.
    • Bile also ==activates lipases.==
  • The intestinal mucosal epithelium has goblet cells that secrete mucus.
  • The secretions of the ==brush border cells of the mucosa== along with the secretions of the goblet cells constitute the ==intestinal juice or succus entericus.==
    • This juice contains a variety of enzymes like disaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases, etc.
  • The mucus along with the bicarbonates from the pancreas ==protects the intestinal mucosa== from acid as well as provides an alkaline medium (pH 7.8) for enzymatic activities.
  • ==Sub-mucosal glands (Brunner’s glands)== also help in this.

  

• The breakdown of biomacromolecules occurs in the duodenum region of the small intestine.

  • The simple substances thus formed are absorbed in the ==jejunum and ileum== regions of the small intestine.
  • The undigested and unabsorbed substances are passed on to the large intestine.
  • No significant digestive activity occurs in the large intestine.

• The functions of the large intestine are

  • ==Absorption of some water, minerals, and certain drugs.==
  • Secretion of mucus which helps in adhering the waste (undigested) particles together and ==lubricating== it for an easy passage.
  • The undigested, unabsorbed substances called feces enter the caecum of the large intestine through the ileocaecal valve, which ==prevents the backflow of the fecal matter.==
  • It is temporarily stored in the rectum till defecation.

• The activities of the gastrointestinal tract are under neural and hormonal control for proper coordination of different parts.

  • The sight, smell, and/or presence of food in the oral cavity can stimulate the secretion of saliva.
  • Gastric and intestinal secretions are also, similarly, stimulated by neural signals.
  • The muscular activities of different parts of the alimentary canal can also be moderated by neural mechanisms, both local and through CNS.

• Hormonal control of the secretion of digestive juices is carried out by local hormones produced by the gastric and intestinal mucosa.

Absorption of Digested Products:

• Absorption is the process by which the end products of digestion pass through the intestinal mucosa into the blood or lymph.

• It is carried out by passive, active, or facilitated transport mechanisms.

  • Small amounts of ==monosaccharides like glucose, and amino acids, and some electrolytes like chloride ions== are generally absorbed by simple diffusion.
  • The passage of these substances into the blood depends upon the ==concentration gradients.==

• However, some substances like glucose and amino acids are absorbed with the help of carrier proteins.

  • This mechanism is called ==facilitated transport.==
  • The transport of water depends upon the ==osmotic gradient.==
  • Active transport occurs against the concentration gradient and hence requires energy.
  • Various nutrients like amino acids, monosaccharides like ==glucose, and electrolytes like Na+== are absorbed into the blood by this mechanism.

• ==Fatty acids and glycerol being insoluble==, cannot be absorbed into the blood.

  • They are first incorporated into small droplets called ==micelles== which move into the intestinal mucosa.
  • They are re-formed into very small ==protein-coated fat globules called the chylomicrons== which are transported into the lymph vessels (lacteals) in the villi.
  • These lymph vessels ultimately release the absorbed substances into the bloodstream.
  • Absorption of substances takes place in different parts of the alimentary canal, like the mouth, stomach, small intestine, and large intestine.

• However, maximum absorption occurs in the small intestine.

  

• The absorbed substances finally reach the tissues which utilize them for their activities.

  • This process is called ==assimilation.==
  • The digestive wastes, solidified into coherent faeces in the rectum initiate a neural reflex causing an urge or desire for its removal.

• The egestion of faeces to the outside through the anal opening (defecation) is a voluntary process and is carried out by a mass peristaltic movement.

Disorders of Digestive System:

• The inflammation of the intestinal tract is the most common ailment due to bacterial or viral infections.
  • The infections are also caused by the parasites of the intestine like tapeworm, roundworm, threadworm, hookworm, pinworm, etc.

Jaundice:

• The liver is affected, and skin and eyes turn yellow due to the deposit of bile pigments.

Vomiting:

• It is the ejection of stomach contents through the mouth.
  • This reflex action is controlled by the vomiting center in the medulla.
  • A feeling of nausea precedes vomiting.

Diarrhea:

• The abnormal frequency of bowel movement and increased liquidity of the faecal discharge is known as diarrhea.
  • It reduces the absorption of food.

Constipation:

• In constipation, the feces are retained within the colon as the bowel movements occur irregularly.

Indigestion:

• In this condition, the food is not properly digested leading to a feeling of fullness.
  • The causes of indigestion are inadequate enzyme secretion, anxiety, food poisoning, overeating, and spicy food.

Protein-energy Malnutrition:

• Dietary deficiencies in proteins and total food calories are widespread in many underdeveloped countries of ==South and Southeast Asia, South America, and West and Central Africa.==
  • This may affect large sections of the population during drought, famine, and political turmoil.
  • This happened in ==Bangladesh during the liberation war and in Ethiopia during the severe drought in the mid-eighties.==
  • PEM affects infants and children to produce ==Marasmus and Kwashiorkor.==
    • ==Marasmus is produced by a simultaneous deficiency of proteins and calories.==
    • It is found in ==infants less than a year== in age if a ==mother’s milk is replaced== too early by other foods which are poor in both ==proteins and caloric valu==e.
      • This often happens if the ==mother has second pregnancy or childbirth== when the older infant is still too young.
    • In Marasmus, protein deficiency ==impairs the growth and replacement of tissue proteins; extreme emaciation of the body and thinning of limbs results, in the skin becoming dry, thin, and wrinkled.==
    • Growth rate and body weight decline considerably.
    • Even growth and development of the brain and mental faculties are impaired.
    • Kwashiorkor is produced by ==protein deficiency unaccompanied by calorie deficiency.==
    • It results from the ==replacement of a mother’s milk with a high-calorie low protein diet== in a child more than one year in age.
    • Like marasmus, kwashiorkor shows ==wasting of muscles, thinning of limbs, failure of growth, and brain development.==
• But unlike marasmus, some fat is still left under the skin; moreover, extensive edema and swelling of body parts are seen

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